1. Field of the Invention
The present invention relates to an ATM-based private communication network which is suitable for applications where propagation delays cannot be ignored for quality speech communications.
2. Description of the Related Art
In a known ATM-based private network, communication sites such as business locations each include a PBX switch, an ATM switch and a cell assembly/disassembly unit connected between these switches. The ATM switches of source and destination sites are interconnected via one or more ATM switches or by way of their associated cell assembly/disassembly unit and the PBX switch. However, if no direct, short-cut path exists between the source and destination station, speech signals over the transit path experience long processing delay associated with cell assembly and disassembly, which is typically 30 milliseconds for each station. If voice-band compander (compression/expansion) is used for PCM speech signals, an extra amount of delays would be added. In addition to the end-to-end propagation delay, there is a delay variation factor that must be taken into account for quality speech communication. Thus, it is important to avoid routes having long delays and delay variations. Although delay variations may be absorbed by the use of buffers, best route selection is still a problem.
It is therefore an object of the present invention to provide an ATM-based private communication network in which processing delays and delay variations are reduced and which allows best routes to be selected.
According to a first aspect of the present invention, there is provided a communication network comprising source and destination stations and a transit station interconnected by transmission mediums. Each of the stations comprises a PBX switch, an ATM switch connected via the transmission mediums to ATM switches of other stations, and a cell assembly/disassembly unit connected between the PBX switch and the ATM switch, the unit controlling the PBX switch and the ATM switch to transfer a voice signal over a short-cut connection between the source and destination stations and transfer control signals through the PBX switch of the transit station.
According to a second aspect, the present invention provides a cell assembly/disassembly unit for interfacing between a PBX switch and an ATM switch connected to another ATM switch by a transmission medium. The cell assembly/disassembly unit comprises means for formulating a trunk identification number in an ATM cell according to ATM cell format and transmitting the cell to the network and means for receiving ATM cells from the network and extracting source and destination trunk identification numbers from the received ATM cells. A mapping table provides mapping of a plurality of trunk terminals of the PBX switch to a plurality of corresponding VPIs/VCIs. A controller is provided for looking up the mapping table with the received source and destination trunk identification numbers to obtain a VPI/VCI value and formulating a command message in an ATM cell containing the VPI/VCI value and supplying the ATM cell to the ATM switch.
According to a third aspect, the present invention provides a cell assembly/disassembly unit for interfacing between a PBX switch and an ATM switch, comprising a bit insertion circuit connected to an incoming trunk terminal of the PBX switch for inserting each bit of a bit sequence identifying the incoming trunk terminal into a least significant bit position of digital signal at the incoming trunk terminal, an extraction circuit connected to an outgoing trunk terminal of the PBX switch for extracting the bit sequence, and a controller connected to the extraction circuit for establishing a relationship between a call and the incoming and outgoing trunk terminals when a connection is established in the PBX switch between the incoming and outgoing trunk terminals.
According to a fourth aspect of the present invention, there is provided a method for a communication network, wherein the network includes a source station, a transit station and a destination station interconnected by transmission mediums, each of the stations including a PBX switch, an ATM switch and a cell assembly/disassembly unit connected between the PBX and ATM switches. According to the method, a call setup message is sent from the PBX switch of the source station to the PBX switch of the transit station, and connections are established in the PBX switch of the transit station according to the call setup message, and a call setup message is sent from the PBX switch of the transit station to the PBX switch of the destination station. Source and destination trunk identification numbers of the source and destination stations associated with the call setup message are exchanged so that all of the stations share the source and destination trunk identification numbers. A short-cut connection is established between the ATM switch of the source station and the ATM switch of the destination station according to the source and destination trunk identification numbers.